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NCERT Solutions Class 11 Biology Chapter 8 - Cell: The Unit of Life
NCERT Solutions for Class 11 Biology Chapter 8 (Cell: The Unit of Life) explore the fundamental building block of all living organisms. Every biological process, from digestion to thinking, is ultimately a result of cellular activity. This chapter transitions from the study of whole organisms to the microscopic world of the Cell Theory, membranes, and specialized organelles.
NCERT Solutions for Class 11 Biology Chapter 8 by ALLEN are designed by expert faculty to simplify the complex architecture of the cell. The solutions focus on the structural and functional details of organelles like Mitochondria, Chloroplasts, and the Nucleus, making it easier for students to understand how a cell maintains its "life" through compartmentalization. For NEET aspirants, this is a "must-master" chapter. It serves as the foundation for Genetics, Biotechnology, and Human Physiology. These solutions provide the technical precision needed to understand the Fluid Mosaic Model and the differences between plant and animal cells, which are high-weightage topics in medical entrance exams.
1.0Class 11 Biology Chapter 8 Cell: The Unit of Life: Key Concepts
This chapter examines the structure and functional organization of cells across different kingdoms. The key lessons include:
- What is a Cell?: Understanding cells as the basic structural and functional units of life and the history of Cell Theory proposed by Schleiden and Schwann (and modified by Virchow).
- Prokaryotic Cells: Analyzing the structure of bacteria, including the cell envelope, ribosomes, and Inclusion Bodies.
- Eukaryotic Cells: A detailed look at the complex internal organization of higher cells.
- Cell Membrane: Understanding the Fluid Mosaic Model proposed by Singer and Nicolson.
- Cell Wall: Studying the structure of the cell wall in plants, fungi, and algae.
- Endomembrane System: Exploring the coordinated functions of the Endoplasmic Reticulum (ER), Golgi apparatus, Lysosomes, and Vacuoles.
- Mitochondria and Plastids: Analyzing the "Powerhouse" of the cell and the different types of plastids like Chloroplasts, Chromoplasts, and Leucoplasts.
- Ribosomes and Cytoskeleton: Understanding protein factories and the structural framework (Microtubules, Microfilaments).
- Cilia, Flagella, and Centrosomes: Studying the motor and division apparatus of the cell.
- Nucleus: Detailed study of the nuclear envelope, nucleoplasm, nucleolus, and Chromatin structure.
- Microbodies: Brief overview of membrane-bound minute vesicles.
2.0NCERT Solutions Class 11 Biology Chapter 8 -Cell: The Unit of Life: Detailed Solutions
- Which of the following is not correct? (a) Robert Brown discovered the cell. (b) Schleiden and Schwann formulated the cell theory. (c) Virchow explained that cells are formed from pre-existing cells. (d) A unicellular organism carries out its life activities within a single cell. Ans. (a) Robert Brown discovered the cell.
- New cells generate from (a) bacterial fermentation (b) regeneration of old cells (c) pre-existing cells (d) abiotic materials. Ans. (c) pre-existing cells
- Match the following.
Ans. a - (ii); b - (iii); c - (i).
- Which of the following is correct? (a) Cells of all living organisms have a nucleus. (b) Both animal and plant cells have a well defined cell wall. (c) In prokaryotes, there are no membrane bound organelles. (d) Cells are formed de novo from abiotic Ans. (c) In prokaryotes, there are no membrane bound organelles.
- What is a mesosome in a prokaryotic cell? Mention the functions that it performs. Ans. • Mesosome is a membranous structure in prokaryotic cell, which is formed by the extensions of the plasma membrane into the cell in form of vesicles, tubules and lamellae.
- Mesosomes are equal to mitochondria in eukaryotes, as they perform aerobic cellular respiration in prokaryotes.
- It helps in DNA replication and distribution of genetic material to daughter cells.
- Mesosomes also help in respiration, increase the surface area of the plasma membrane and enzymatic content.
- How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane? Ans. Neutral solutes move across the membrane by the process of simple diffusion along the concentration gradient i.e., from higher concentration to the lower concentration. Polar molecules cannot pass through the nonpolar lipid bilayer, they require carrier protein of the membrane to facilitate their transport across the membrane.
- Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both? Ans. Mitochondria and chloroplast are double membrane bound organelles. Mitochondria : Mitochondria are cylindrical or sausage shaped cell organelles and contains two membranes, outer and inner. The inner compartment is filled with a dense homogenous substance called the matrix containing single circular DNA, RNA, 70'S ribosomes, and the components required for the synthesis of proteins and outer membrane forms the continuous limiting boundary of the organelle. Inner membrane forms number of infoldings called the cristae which increases the surface area. Oxysomes are present on inner mitochondrial membrane. Mitochondria are semiautonomous organelles, i.e., have their own DNA and ribosomes.
Functions of Mitochondria:
- Mitochondria are the site of aerobic respiration.
- They produce cellular energy in the form of ATP, hence they are called power henses of the cell.
- Mitochondrial matrix contains enzymes for the synthesis of fatty acids.
- Synthesis of many amino acids takes place here Chloroplast : They are green coloured plastids which are disc shaped. The space limited by inner membrane of chloroplast is called as stroma. Stroma has organised flattened membranous sacs called the thylakoids. Thylakoids are arranged in stacks like the piles of coins called grana. Matrix of a chloroplast contains DNA, RNA, ribosomes and enzymes. Chloroplast is also a semiautonomous organelle.
Structure of a chloroplast
Functions of Chloroplast :
- Photosynthesis is performed by chloroplast.
- Chloroplast contain chlorophyll and carotenoid pigments.
- Maintains balance of concentration in the air.
- Keeps oxygen balance constant in atmosphere by liberating into the atmosphere, used during respiration and combustion.
- What are the characteristics of prokaryotic cells? Ans. Characteristics of prokaryotic cells are as follows:
- Prokaryotes have 70'S ribosomes.
- DNA is naked and lies coiled in cytoplasm. It is not covered by nuclear membrane and is termed as nucleoid or genomic DNA.
- In addition to the genomic DNA (the single chromosome/circular DNA) many bacteria have small circular DNA outside the genomic DNA called plasmid.
- Nuclear components, like, nuclear envelope, nucleolus, nucleoplasm are absent.
- Cell wall is present in bacteria and cyanobacteria, but absent in mycoplasma.
- Multiplies by asexual reproduction.
- Transcription and translation takes place in cytoplasm.
- Multicellular organisms have division of labour. Explain.
Ans. Division of labour is differentiation of certain components or parts to perform different functions for increased efficiency and higher survival.
- Multicellular organisms often possess millions of cells. Various cells are grouped together to form specific tissue, organ or organ system, with each specialised to perform particular function.
- Every cell of a multicellular organism cannot obtain food from outside. The organism requires a system for obtaining food, its digestion and distribution. Therefore, a digestive system and system of transport are also required.
- Certain cells of the body take over the function of reproduction.
- Others take part in repair and replacement
- For optimum functioning of cells also requires an internal favourable environment.
- Cell is the basic unit of life. Discuss in brief.
Ans. Cell is fundamental, structural and functional unit of life, as no living organism can have life without being cellular.
- An organism is either made of single cell (unicellular) or many cells (multicellular).
- In unicellular organism, single cell is capable of independent existence and perform all essential functions of life, while in multicellular organism, each group of cells is specialised for specific function.
- Life passes from one generation to the next in form of cells, and new cell always arise from division of pre-existing cells.
- Cells are totipotent, i.e., single cell has ability to form whole organism.
- The activities of an organism are sum total of activities of its cells, therefore, cell is the basic unit of life.
- What are nuclear pores? State their function.
Ans. The nuclear envelope is interrupted by minute nuclear pores, at a number of places, which are produced by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
- Both lysosomes and vacuoles are endomem-brane structures yet they differ in terms of their functions. Comment.
Ans. Organelles of endomembrane system such as lysosome and vacuoles function in close coordination with one another but are specialised to perform different functions. Lysosomes breakdown the ageing and dead cells, they help in digestion of food as they contain hydrolytic digestive enzymes. They are involved in cell division also. Vacuoles on other hand, help in excretion and osmoregulation in Amoeba (contractile vacuole).
- Describe the structure of the following with the help of labelled diagrams. (i) Nucleus (ii) Centrosome
Ans.
(i) Nucleus: Nucleus is double membrane bound principle cell organelle which contains all genetic information for controlling cellular metabolism and transmission of genetic information.
Nucleus is differentiated into following parts :
(a) Nuclear envelope : It is a double membrane bound envelope that surround the nucleus and separates the latter from the cytoplasm. (b) Nucleoplasm : The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis. (c) Chromatin : It has the ability to get stained with certain basic dyes. It is known to be the hereditary DNA protein fibrillar complex. The chromatin fibres are distributed throughout the nucleoplasm.
(ii) Centrosome: Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other. They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked. The hub of centriole is connected with tubules of the peripheral triplets by radial spokes made of protein.
- What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes. Ans. A chromosome consists of two identical halves, the chromatids held together at one point called the centromere. The centromere is also called as primary constriction. On its side a disc shaped structure called kinetochore is present. Chromosomes are classified into four types according to position of centromere on the chromosome. (i) Metacentric chromosome : In this chromosome, centromere is in the middle and the two arms are almost equal in length. (ii) Submetacentric chromosome : The centromere is slightly away from middle point so one arm is slightly shorter than the other. (iii) Acrocentric chromosome : The centromere is near the end and one arm is extremely short and other arm is extremely long. (iv) Telocentric chromosome : Centromere is at the tip of chromosome. These chromosomes are not present in humans.
3.0Key Features of NCERT Solutions for Class 11 Biology Chapter 8
- Structural-Functional Mapping: The solutions clearly link the structure of each organelle (e.g., the folded cristae of mitochondria) to its function (increasing surface area for ATP production).
- Fluid Mosaic Model Clarity: Detailed explanations of the lipid bilayer and the "mosaic" of proteins, helping students visualize how molecules move across membranes.
- Comparative Biology Focus: Clear tables comparing Prokaryotic vs. Eukaryotic cells and Plant vs. Animal cells, which are essential for board exams.
- Semi-Autonomous Organelle Logic: Expertly explained reasoning behind why Mitochondria and Chloroplasts are called semi-autonomous (presence of their own DNA and 70S ribosomes).
- Prepared by Expert Faculty: Prepared by ALLEN subject experts, these solutions ensure biological accuracy and strict alignment with the latest NCERT and NEET syllabus.
- Diagram-Oriented Precision: The solutions emphasize the "9+2" and "9+0" microtubule arrangements, which are common identification-based questions in competitive exams.
- Complete Coverage of NCERT Exercises: Every question is answered with the depth required to understand the cell as a living, dynamic system.